Hear Carry Over

Description

Hear Carry Over (HCO) is a service feature standardized by 3GPP that facilitates an alternating mode of communication where a user can receive speech audio and transmit text data in a sequential or parallel manner. The core operational principle involves the terminal device (e.g., a mobile phone or assistive device) managing two distinct media streams: a downlink speech channel and an uplink text channel. The user interface and network protocols coordinate to allow the user to listen to incoming speech and then compose and send a text response, creating a conversational turn-taking pattern. This is often implemented in conjunction with services like Total Conversation or text telephony (TTY) over IP networks, where the Real-time Transport Protocol (RTP) carries the speech and text media, potentially using RTP payload formats for text. The network's IP Multimedia Subsystem (IMS) typically provides the service control, with Session Initiation Protocol (SIP) used for session establishment and feature negotiation. The terminal must support specific codecs and buffering mechanisms to handle the timing and synchronization between the audio and text streams, ensuring the conversation feels natural and responsive. From a network perspective, HCO sessions require appropriate Quality of Service (QoS) handling, as the real-time speech stream is delay-sensitive, while the text stream, though real-time, can tolerate slightly different latency profiles. The feature relies on the underlying packet-switched network's ability to support concurrent media flows with different characteristics, a capability inherent in LTE and 5G networks.

Purpose & Motivation

HCO was introduced to address the need for accessible telecommunications for individuals with hearing or speech disabilities. Prior to such standardized features, communication options were limited, often relying on separate, dedicated text telephony devices that were not integrated into mainstream mobile services. The motivation was to leverage the capabilities of IP-based mobile networks (like those defined from 3GPP Release 10 onwards) to provide integrated, real-time conversational services that combine audio and text. This solves the problem of enabling two-way communication where one party may primarily use text and the other speech, a common scenario in relay services or direct conversations between hearing and non-hearing users. By standardizing HCO, 3GPP ensured interoperability between user equipment and network services, promoting widespread adoption and inclusion. It addressed the limitations of previous circuit-switched text telephony, which was often slow, had poor sound quality, and was not seamlessly integrated with modern multimedia services. HCO, as part of a suite of accessibility features, allows mobile networks to fulfill regulatory and social responsibilities for universal service.

Key Features

• Alternating reception of speech and transmission of text
• Support for real-time text (RTT) transport over IP
• Integration with IMS-based service delivery
• Session management via SIP and SDP negotiation
• Utilization of RTP for media carriage with appropriate payload formats
• Terminal-based buffering and user interface coordination for turn-taking

Evolution Across Releases

Defining Specifications